The invention relates to heating and forming metallic sheet structure and more particularly to an improved tooling system and method for forming and/or bonding metallic structures. Normally, the forming is superplastic forming and the bonding is diffusion bonding.
Superplasticity is the property of a material to develop unusually high tensile elongations and plastic deformation with a reduced tendency towards necking within a limited temperature and strain rate range. This property is only exhibited by a limited number of metals and their alloys, such as titanium and its alloys, and aluminum and its alloys.
In superplastic forming a workpiece is heated until it becomes superplastic, after which differential pressure is applied causing the workpiece to stretch and form into a die cavity. The forming process is a complex one with critical parameters (time, temperature, and pressure) controlling the rate of stretching. Necking and ruptures are the direct result of exceeding the narrow tolerances of these parameters.
The process of superplastic forming (SPF) is disclosed in U.S. Pat. No. 3,934,441 to Hamilton et. al., entitled "Controlled Environment Superplastic Forming of Metals," which is incorporated into this specification by reference. The patent discloses an apparatus for the superplastic forming of reactive metals within a controlled environment, and a number of tool sealing techniques.
Bonding is the joining of similar or dissimilar metal surfaces together by placing the surfaces in direct contact and applying high pressures and heat. Although deformation bonding or solid state bonding are commonly used, diffusion bonding is preferred. Diffusion bonding (DB), as used herein, is the metallurgical joining of surfaces by applying heat and pressure for a sufficient time to cause commingling of the molecules at the joint interface. Diffusion bonding requires the intimate contact of clean mating surfaces to allow the inter-molecular attractive forces to interact.
In many applications superplastic forming is combined with diffusion bonding (SPF/DB) because of the similarities in process pressures and temperatures. The SPF/DB process is disclosed in U.S. Pat. No. 3,920,175 entitled "Method for Superplastic Forming of Metals with Concurrent Diffusion Bonding," and U.S. Pat. No. 3,927,817, entitled "Method for Making Metallic Sandwich Structures," both to Hamilton, et al., and both being incorporated herein by reference.
U.S. Pat. No. 4,331,284 by Schulz, et. al. entitled "Method of Making Diffusion Bonded and Superplastically Formed Structures" discloses a tooling system. The upper forming member has a circular protrusion that surrounds the fluid line inlet and continuous frame protrusions that surrounds the perimeter of the sheets. Upon closing, the protrusions form indentations in the sheets, thereby providing circular seals around the fluid inlet lines and peripheral seals around the perimeter of the sheets. These seals were used because standard resilient seal materials tend to:
1. break down while subjected to high process temperatures and high pressure differentials; PA0 2. be incompatible with sheet materials; and PA0 3. contaminate the inert atmosphere at process temperatures.
However, these protruding seals may crush the inflation tubes and extrude the sheet material. In addition, leakage can occur during forming, thereby contaminating the forming chamber and damaging the formed structure.